Seebeck Effect
Thermocouples were discovered in 1821 when Thomas J. Seebeck observed the phenomenon of thermoelectricity - the Seebeck Effect. When two dissimilar metals are joined creating two junctions and those "sensing junctions" are at different temperatures - an electric current flows through the created circuit. The emf is proportional to the temperature difference and is measured in mV.
Thermocouple measurements are deduced from the millivolt reading and the known reference junction temperature. A signal converter must make the millivolt reading and compare the millivolt value of the reference junction at a known temperature to be able to make the temperature measurement.
Hot or
Hot or
Measuring
Junction
Cold or
Cold or
Reference
Junction
Selection of a Thermocouple
Factors to consider when selecting a thermocouple:
· Temperature Range
· Media
· Required Response Time
· Accuracy
· Measurement System Installation
· Cold Junction Compensation
· Leadwire Compatibility
Standard Thermocouples
Any combination of dissimilar metals will create the
Seebeck Effect; however eight wire combinations are predominately used in thermocouples in the
United States. They are types: J, K, T, E, R, S, B
& N.
Refer to the chart on the following page for characteristics of each type.
Thermocouples are designated by capital letters that refer to their composition according to the American National Standards
Institute (ANSI). They have been chosen for stability of EMF, ability to be replicated, mechanical and chemical properties, and costs.
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Standard Thermocouple Characteristics
Type Wire Materials
J Iron (+)
Constantan (-)
Wire Insulation Recommended
Coloring Atmosphere
White
Red
Vacuum, Inert,
Oxidizing or Reducing
K
T
E
N
R
S
B
Chromel (+)
Alumel (-)
Copper (+)
Constantan (-)
Chromel (+)
Constantan (-)
Nicrosil (+)
Nisil (-)
Platinum-13%/
Rhodium (+)
Platinum-10%/
Rhodium (+)
Platinum (-)
Platinum-30%/
Rhodium (+)
Platinum (-)
Yellow
Red
Blue
Red
Purple
Red
Orange
Red
Black
Red
Black
Red
Grey
Red
Features
Inert or Oxidizing
Vacuum, Inert,
Oxidizing or Reducing
Inert, Oxidizing or
Reducing
Inert or Oxidizing
Inert or Oxidizing
Inert or Oxidizing
Inert or Oxidizing
Most commonly used. Iron wire may rust or become brittle at sub-zero temperatures. If unprotected, iron wire may be attacked by ammonia, nitrogen and hydrogen atmospheres.
Type J should not be used in sufurous atmospheres about 540°C.
Generally used to measure high temperatures to
2300°F. Should not be used for accurate measurement to below 900°F or after prolonged exposure above 1400°F. Short life in alternately oxidizing and reducing atmospheres.
Commonly used for sub-zero to 700°F temperature. Superior corrosion resistance.
Preferred to type J for sub-zero applications due to copper’s higher moisture resistance, as compared to iron.
Highest EMF output of any standard metallic thermocouple. Wires are not subject to corrosion at sub-zero temperatures.
Developed for oxidation resistance and EMF stability superior to those of type K thermocou ples at elevated temperatures. Popular in
Europe.
Good linearity at high temperatures. Should always be protected from contamination insulation should be silica free.
Widely used in laboratories as a standard for calibration of base metal thermocouples and other temperature sensing instruments.
For use between 1000 and 3175°F. As with other platinum thermocouples, protection from contamination with silica-free insulation is required.
Typical Response Time for MgO Constructed Thermocouples
Thermocouple O.D. (inch) Measuring Junction Typical Response (seconds)
1/50
1/50
1/25
1/25
1/16
1/16
1/8
1/8
3/16
3/16
1/4
1/4
1/4
Grounded 0.05
Ungrounded 0.15
Grounded 0.1
Ungrounded 0.3
Grounded 0.2
Ungrounded
Grounded
Ungrounded
Grounded
Ungrounded
0.5
0.7
1.3
1.1
2.2
Grounded
Ungrounded
Exposed
2.0
4.5
0.1
44 Rev 0204

Junction Type/Description
Ungrounded Junction
Thermocouple junction is insulated from sheath and ground.
Provides better performance under noisy conditions. Slower response time than grounded junction thermocouples.
Grounded Junction
Thermocouple junction is an integral part of the sheath. Provides fast response time and is least expensive construction. Susceptible to ground loops and noise interference.
Exposed Junction
Thermocouple junction extends beyond sheath. Provides fastest response. Not suitable for pressurized or corrosive environments.
Moisture will affect performance.
Initial Thermocouple Tolerances
Thermocouple
N
R
S
B
J
K
T
E
Range
32 to 1400°F
-328 to 2300°F
-328 to 700°F
-328 to 1600°F
-454 to 2372°F
32 to 2700°F
32 to 2700°F
32 to 3100°F
ANSI Standard Limits of Error ANSI Special Limits of Error
(Whichever is greater) (Whichever is greater)
Weed Standard
±4.0°F or 0.75%
±4.0°F or 0.75%
±1.8°F or 0.75%
±3.0°F or 0.50%
±4.0°F or 0.75%
±2.7°F or 0.25%
±2.7°F or 0.25%
±0.50%
±2.0°F or 0.40%
±2.0°F or 0.40%
±0.9°F or 0.40%
±1.8°F or 0.40%
±2.0°F or 0.40%
±1.1°F or 0.10%
±1.1°F or 0.10%
±0.25%
Weed Instrument Company uses only ANSI Special Limits of Error in its manufacturing of thermocouples.
Rev 0204 45